[Regeneration of murine fibroblasts, L-929, following fractionated irradiation].

L-cells in culture were irradiated with single and fractionated doses of 200 kV X-rays. Dose effect curves were determined using the colony forming capacity as the criterion of the radiation effect. After an initial dose of 330 rd and a 3 h to 16 h incubation at 37 degrees C a restitution of the shoulder of the dose effect curves was observed (Elkind repair). 16 hours after preirradiation the dose effect curve showed a statistically significant increase of n and Dq as compared to the dose effect curve after single irradiations showing that the cells have become more radioresistant by the radiation pretreatment. The curve of the colony forming ability after a total dose of 660 rd given in two equal doses of 330 rd had an oscillating shape depending on the fractionation interval. This type of curve can be interpreted by the restitution progression model.